This invention relates generally to ribbed pipe that is formed by helically coiling strip material and welding the two edges of the strip as they are held in abutting relation. A process for forming such pipe is described in U.S. Pat. No. 3,601,570. The shortcomings of the process described in that patent are several. First, it was difficult to maintain a high speed weld in forming the pipe and/or yet achieve 100% weld penetration; secondly, it was difficult to register the two side edges and maintain those edges in the same plane because of strip camber. For these reasons, many manufacturing problems were presented, resulting in either a weak weld joint or a very slow process of manufacture--and usually both.
Helically welded pipe has also been formed by overlapping the two side edges. But this lap-weld process also results in many failures since many foreign particles, such as scale, are produced in the normal course of manufacture. Moreover, it is well-known that a butt weld connection provides the best form of fusion (or forge) provided that the required heat is applied to the abutting edges and sufficient pressure is applied at the time of fusion.